Immunity to M. tuberculosis is characterized by some basic features; specifically sensitized T lymphocytes mediates protection, and the most important mediator molecule seems to be interferon gamma (IFN-γ).
M. tuberculosis holds, as well as secretes, several proteins of potential relevance for the generation of a new TB vaccine. In 1998 Cole et al. published the complete genome sequence of M. tuberculosis and predicted the presence of approximately 4000 open reading frames1. However importantly, this sequence information cannot be used to predict if the DNA is translated and expressed as proteins in vivo. The genome sequence has been used extensively to design DNA arrays for RNA expression analysis and in proteome studies to identify expressed proteins. Even with the vaste amount of expression data and the significant improvement of in silico prediction tools it is still not possible to predictic with certainty that a given sequence will encode an immunogenic molecule. The only way to determine if a molecule is recognized by the immune system during or after an infection with M. tuberculosis is to produce the given molecule and test it in an appropriate assay as described herein.
Currently there are several new TB vaccines in clinical trials. However, they are primarily classical preventive vaccines based on a limited number of antigens expressed in the early stage of infection. As a direct consequence of the expression dynamic the epitope pattern that is presented to T cells changes radically over time—implicating how new vaccines should be designed. E.g. for the transiently expressed early antigen, Ag85B, two independent T cell transfer studies have shown that 3-4 weeks after infection, Ag85B is no longer being presented to T cells and as a result there is no Ag85B specific production of cytokine's, chemokine's etc. at this or later time points of the infection2,3. Thus, it is of limited value for a chronic disease that establish long-term co-existence with the host to vaccinate and induce memory T cells specific for epitopes in proteins that are only expressed during a brief period of the infection.
For vaccine development it is therefore vital to identify antigens that are highly expressed in the later stage of infection and among these select those that are immunogenic and can contribute to protection and include this special subset of proteins in TB vaccines. By doing so it is not only possible to improve vaccine potency and epitope coverage but also target latent infections.
Mycobacteria secretion systems are responsible for the export of proteins into the extracellular environment. The 6-kDa early secretory antigenic target of Mycobacterium tuberculosis (ESAT-6) and the 10-kDa culture filtrate antigen (CFP-10), are proteins secreted by the ESAT-6 secretion system 1 (ESX-1) and are among the most immunodominant M. tuberculosis (MTB) antigens. These attributes makes them important for tuberculosis (TB) vaccine development. Based upon this knowledge we tested other ESX-1 associated proteins as potential TB vaccine antigens.